Persistent organic pollutants (POPs): state of the science K.C. Jones a, *, P. de Voogt b a Environmental Science Department, Institute of Environmental and Natural Sciences, Lancaster University, Lancaster, LA1 4YQ, UK b Department of Environmental and Toxicological Chemistry, Amsterdam Research Institute for Substances in Ecosystems (ARISE), University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, Netherlands Received 15 November 1998; accepted 22 March 1999 Abstract The environmental chemistry and ecotoxicology of persistent organic pollutants (POPs) are fascinating areas of scienti®c research. Our objective in this paper is to provide a brief, focussed overview of what constitutes a POP, highlight the harmful eects they may have on biota, make some comments on their environmental sources and analysis, their environmental trends and pro- cesses, their movement through foodchains and highlight some important regional-and global-scale environmental transport issues. Finally, we oer some personal thoughts on some current and future areas of scienti®c enquiry on POPs. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Persistent organic pollutant, POP; Sources; Air±surface exchange; Biota; Foodchains 1. What are Persistent Organic Pollutants (POPs) and their properties? There are many thousands of POP chemicals, often coming from certain series or `families' of chemicals (e.g there are theoretically 209 dierent polychlorinated biphenyls, diering from each other by level of chlor- ination and substitution position). POPs are persistent in the environment, having long half-lives in soils, sedi- ments, air or biota. There seems to be no consensus of opinion about how long the half-life in a given media should be for the term `persistent' to be conferred; however, in practice a POP could have a half-life of years or decades in soil/sediment and several days in the atmosphere. A recent approach has considered de®ning `persis- tence in the environment' operationally from a model- calculated overall residence time at steady state in a multimedia environment (Webster et al., 1998). POPs are typically `water-hating' and `fat-loving' chemicals, i.e. hydrophobic and lipophilic. In aquatic systems and soils they partition strongly to solids, notably organic matter, avoiding the aqueous phase. They also partition into lipids in organisms rather than entering the aqueous milieu of cells and become stored in fatty tissue. This confers persistence on the chemical in biota since metabolism is slow and POPs may there- fore accumulate in foodchains. Importantly, POPs have the propensity to enter the gas phase under environmental temperatures. Hence, they may volatilise from soils, vegetation and water bodies into the atmosphere andÐbecause of their resis- tance to breakdown reactions in airÐtravel long dis- tances before being re-deposited. The cycle of volatilisation and deposition may be repeated many times, with the result that POPs could accumulate in an area far removed from where they were used or emitted. In the atmosphere itself, POPs can partition between particles and aerosols depending on ambient tempera- ture and the physico-chemical properties of the chemical. In summary, then, the combination of stability and propensity to form a gas under appropriate environ- mental conditions means that POPs are subject to long- range atmospheric transport (see later). The combina- tion of resistance to metabolism and lipophilicity means that POPs will accumulate in foodchains. Amongst the important classes of POP chemicals are many families of chlorinated (and brominated) aro- matics, including polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins and-furans (PCDD/ 0269-7491/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0269-7491(99)00098-6 Environmental Pollution 100 (1999) 209±221 www.elsevier.com/locate/envpol * Corresponding author. Fax:+44-1524-593985. E-mail address: k.c.jones@lancaster.ac.uk (K.C. Jones)